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PostGIS ajoute le support d'objets géographique à la base de données PostgreSQL. En effet, PostGIS "spatialise" le serverur PostgreSQL, ce qui permet de l'utiliser comme une base de données SIG.

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source: trunk/workshop-foss4g/projection.rst @ 11

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[1]1.. _projection:
2
3Section 15: Projecting Data
4===========================
5
6The earth is not flat, and there is no simple way of putting it down on a flat paper map (or computer screen), so people have come up with all sorts of ingenious solutions, each with pros and cons. Some projections preserve area, so all objects have a relative size to each other; other projections preserve angles (conformal) like the Mercator projection; some projections try to find a good intermediate mix with only little distortion on several parameters. Common to all projections is that they transform the (spherical) world onto a flat cartesian coordinate system, and which projection to choose depends on how you will be using the data.
7
8We've already encountered projections when we :ref:`loaded our nyc data <loading_data>`.  (Recall that pesky SRID 26918).  Sometimes, however, you need to transform and re-project between spatial reference systems. PostGIS includes built-in support for changing the projection of data, using the :command:`ST_Transform(geometry, srid)` function. For managing the spatial reference identifiers on geometries, PostGIS provides the :command:`ST_SRID(geometry)` and :command:`ST_SetSRID(geometry, srid)` functions.
9
10We can confirm the SRID of our data with the :command:`ST_SRID` command:
11
12.. code-block:: sql
13
14  SELECT ST_SRID(the_geom) FROM nyc_streets LIMIT 1;
15 
16::
17
18  26918
19 
20And what is definition of "26918"? As we saw in ":ref:`loading data section <loading_data>`", the definition is contained in the ``spatial_ref_sys`` table. In fact, **two** definitions are there. The "well-known text" (:term:`WKT`) definition is in the ``srtext`` column, and there is a second definition in "proj.4" format in the ``proj4text`` column.
21
22.. code-block:: sql
23
24   SELECT * FROM spatial_ref_sys WHERE srid = 26918;
25   
26In fact, for the internal PostGIS re-projection calculations, it is the contents of the ``proj4text`` column that are used. For our 26918 projection, here is the proj.4 text:
27
28.. code-block:: sql
29
30  SELECT proj4text FROM spatial_ref_sys WHERE srid = 26918;
31 
32::
33
34  +proj=utm +zone=18 +ellps=GRS80 +datum=NAD83 +units=m +no_defs
35 
36In practice, both the ``srtext`` and the ``proj4text`` columns are important: the ``srtext`` column is used by external programs like `GeoServer <http://geoserver.org>`_, `uDig <udig.refractions.net>`_, and `FME <http://www.safe.com/>`_  and others; the ``proj4text`` column is used internally.
37
38Comparing Data
39--------------
40
41Taken together, a coordinate and an SRID define a location on the globe. Without an SRID, a coordinate is just an abstract notion. A “Cartesian” coordinate plane is defined as a “flat” coordinate system placed on the surface of Earth. Because PostGIS functions work on such a plane, comparison operations require that both geometries be represented in the same SRID.
42
43If you feed in geometries with differing SRIDs you will just get an error:
44
45.. code-block:: sql
46
47  SELECT ST_Equals(
48           ST_GeomFromText('POINT(0 0)', 4326),
49           ST_GeomFromText('POINT(0 0)', 26918)
50           );
51
52::
53
54  ERROR:  Operation on two geometries with different SRIDs
55  CONTEXT:  SQL function "st_equals" statement 1
56 
57
58.. note::
59
60   Be careful of getting too happy with using :command:`ST_Transform` for on-the-fly conversion. Spatial indexes are built using SRID of the stored geometries.  If comparison are done in a different SRID, spatial indexes are (often) not used. It is best practice to choose **one SRID** for all the tables in your database. Only use the transformation function when you are reading or writing data to external applications.
61
62
63Transforming Data
64-----------------
65
66If we return to our proj4 definition for SRID 26918, we can see that our working projection is UTM (Universal Transverse Mercator) of zone 18, with meters as the unit of measurement.
67
68::
69
70   +proj=utm +zone=18 +ellps=GRS80 +datum=NAD83 +units=m +no_defs
71
72Let's convert some data from our working projection to geographic coordinates -- also known as "longitude/latitude".
73
74To convert data from one SRID to another, you must first verify that your geometry has a valid SRID. Since we have already confirmed a valid SRID, we next need the SRID of the projection to transform into. In other words, what is the SRID of geographic coordinates?
75
76The most common SRID for geographic coordinates is 4326, which corresponds to "longitude/latitude on the WGS84 spheroid". You can see the definition at the spatialreference.org site:
77
78  http://spatialreference.org/ref/epsg/4326/
79 
80You can also pull the definitions from the ``spatial_ref_sys`` table:
81
82.. code-block:: sql
83
84  SELECT srtext FROM spatial_ref_sys WHERE srid = 4326;
85 
86::
87
88  GEOGCS["WGS 84",
89    DATUM["WGS_1984",
90      SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],
91      AUTHORITY["EPSG","6326"]],
92    PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],
93    UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],
94    AUTHORITY["EPSG","4326"]]
95
96Let's convert the coordinates of the 'Broad St' subway station into geographics:
97
98.. code-block:: sql
99
100  SELECT ST_AsText(ST_Transform(the_geom,4326))
101  FROM nyc_subway_stations
102  WHERE name = 'Broad St';
103 
104::
105
106  POINT(-74.0106714688735 40.7071048155841)
107
108If you load data or create a new geometry without specifying an SRID, the SRID value will be -1.  Recall in :ref:`geometries`, that when we created our ``geoemetries`` table we didn't specify an SRID. If we query our database, we should expect all the ``nyc_`` tables to have an SRID of 26918, while  the ``geometries`` table defaulted to an SRID of -1.
109
110To view a table's SRID assignment, query the database's ``geometry_columns`` table.
111
112.. code-block:: sql
113
114  SELECT f_table_name AS name, srid
115  FROM geometry_columns;
116 
117::
118
119          name         | srid 
120  ---------------------+-------
121   nyc_census_blocks   | 26918
122   nyc_neighborhoods   | 26918
123   nyc_streets         | 26918
124   nyc_subway_stations | 26918
125   geometries          |    -1
126
127 
128However, if you know what the SRID of the coordinates is supposed to be, you can set it post-facto, using :command:`ST_SetSRID` on the geometry. Then you will be able to transform the geometry into other systems.
129
130.. code-block:: sql
131
132   SELECT ST_AsText(
133    ST_Transform(
134      ST_SetSRID(geom,26918),
135    4326)
136   )
137   FROM geometries;
138
139Function List
140-------------
141`ST_AsText <http://postgis.org/docs/ST_AsText.html>`_: Returns the Well-Known Text (WKT) representation of the geometry/geography without SRID metadata.
142
143`ST_SetSRID(geometry, srid) <http://postgis.org/docs/ST_SetSRID.html>`_: Sets the SRID on a geometry to a particular integer value.
144
145`ST_SRID(geometry) <http://postgis.org/docs/ST_SRID.html>`_: Returns the spatial reference identifier for the ST_Geometry as defined in spatial_ref_sys table.
146
147`ST_Transform(geometry, srid) <http://postgis.org/docs/ST_Transform.html>`_: Returns a new geometry with its coordinates transformed to the SRID referenced by the integer parameter.
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